1. Field of the Invention
This invention relates to a displacement information measuring apparatus and can be well applied to a velocimeter utilizing the Doppler effect of applying a laser beam, for example, to a moving object or fluid or the like (hereinafter referred to as the "moving object") and detecting the shift of the frequency of scattered light subjected to Doppler shift in conformity with the movement velocity of the moving object to thereby measure the amount of displacement as the information regarding the displacement of the moving object or the movement velocity or the like of the moving object in non-contact.
2. Related Background Art
A laser Doppler velocimeter has heretofore been used as an apparatus for measuring the movement velocity of a moving object in non-contact and highly accurately. The laser Doppler velocimeter is an apparatus for applying a laser beam to a moving object, and measuring the movement velocity of the moving object by utilizing the effect (Doppler effect) of the frequency of scattered light from the moving object shifting in proportion to the movement velocity of the moving object. moving object.
FIG. 1A of the accompanying drawings is an illustration showing an example of the laser Doppler velocimeter according to the prior art.
In FIG. 1A, a laser beam emitted from a laser 1 becomes a parallel light beam 3 by a collimator lens 2, and is divided into two light beams, i.e., transmitted light 5a and reflected light 5b, by a beam splitter 4, and these two light beams are reflected by reflecting mirrors 6a and 6b, whereafter they are applied to a moving object 7 moving at a velocity B at a angle of incident .theta. from different directions. The scattered light from the moving object 7 is detected by a photodetector 9 through a converting lens 8. At this time, the frequencies of the scattered lights by the two light beams are subjected to the Doppler shifts of +.DELTA.f and -.DELTA.f in proportion to the movement velocity V. Here, if the wavelength of the laser beam is .lambda., the change .DELTA.f in the frequency can be expressed by the following expression (1): EQU .DELTA.f=V.multidot.sin(.theta.)/.lambda. (1)
The scattered lights subjected to the doppler shifts of +.DELTA.f and -.DELTA.f interfere with each other and bring about a change of bright and dark on the light receiving surface of the photodetector 9, and the frequency F thereof is given by the following expression (2):
F=2.multidot..DELTA.f=2.multidot.V.multidot.sin(.theta.)/.lambda. (2)
If the frequency F of the photodetector 9 (hereinafter referred to as the "doppler frequency) is measured from expression (2), the movement velocity V of the moving object 7 will be obtained.
A method of improving the S/N ratio of a signal in such a velocimeter is proposed, for example, Japanese Patent Application Laid-Open No. 60-243583. In this publication, there is shown a method of condensing a laser beam applied to a moving object in a non-sensitive direction in velocity detection. Particularly, a cylindrical lens is disclosed as means for condensing a laser beam in a non-sensitive direction in velocity detection.
When in the prior-art velocimeter, a laser beam is to be condensed in the non-sensitive direction in velocity detection by the use of a cylindrical lens, the optical axes of two light beams and the generating-line optical axis of the cylindrical lens must be put together and the assembly must be done strictly. Also, as a matter of course, the number of parts increases correspondingly to the use of the cylindrical lens, and the entire apparatus becomes complicated.
FIG. 1B of the accompanying drawings shows an example of a laser Doppler velocimeter using an optical system which achieves the downsizing of the entire apparatus.
Referring to FIG. 1B, a laser beam emitted from a laser 1 becomes a parallel light beam 3 by a collimator lens 2, and enters a diffraction grating G having a grating pitch d. .+-.first-order diffracted lights R1 and R2 are created by the diffraction grating G, and emerge at a diffraction angle 6 which satisfies the relation that d.multidot.sin(.theta.)=.lambda.. when the two light beams are applied to an object to be measured moving at a velocity V at an angle .theta. of incidence, the scattered light therefrom is detected by a photodetector 9 through a condensing lens 8, and like expression (2), the beat frequency F becomes EQU F=2.multidot.V.multidot.sin(.theta.)/.lambda..
Here, the angle of incidence .theta. is equal to the angle of diffraction .theta. and therefore, there is established the relation that EQU d.multidot.sin(.theta.)=.lambda., (3)
where d is the pitch of the diffraction grating.
From expressions (2) and (3), there is derived the following expression free of the dependency on the wavelength .lambda.: EQU F=2.multidot.V/d (4)
From this, the movement velocity V is found.
By the construction as described above, a construction most compact up to now and free of the wavelength dependency of the laser beam, that is, a construction which eliminates the temperature dependency of a sensor, is made possible.
However, downsizing and low price can be achieved even by the construction as shown in FIG. 1B, but in the case of characteristic which produces not only desired .+-.first-order diffracted lights but also much of unnecessary O-order diffracted light, for example, by the level difference working error or the like of a diffraction grating, there results an interference signal including not the interference between .+-.first-order diffracted lights, but O-order diffracted light, and there has been a case where it is impossible to maintain the S/N of the Doppler signal shown in expression (4) good and the detection accuracy is aggravated.